When Is Your Immune System Strongest: Day or Night?

We know intuitively that travel and disrupted sleep can leave us vulnerable to getting sick. (This is on my mind right now because I’m at the airport, getting ready to head home after a weekend trip during which I got much less sleep than I’m used to.) There are some obvious explanations for this – fatigue leaves us weaker, airplane air is full of germs. But a new study from Yale, published in the journal Immunity[4] (press releases here[5] and here[6]), suggests another possible factor: the strength of your immune system fluctuates with your circadian rhythms, so if you disrupt those rhythms by missing sleep or changing time zone, you leave yourself vulnerable for longer.

Here’s one of the key graphs from the study:

[7]The data shows survival statistics from two groups of mice (along with another two placebo control groups) who were operated on to induce a serious condition called SIRS – systemic inflammatory response syndrome. The only difference between the groups is the time they were infected: those infected during the “active phase” (night for mice, which corresponds to day for humans) tended to die sooner.

The researchers also gave a vaccine to two groups of mice – again, one at night, one during the day. A few weeks later, the mice immunized during the night (their daytime) had stronger immunity as a result. The study attributes both of these effects to the circadian rhythms of “Toll-like receptor 9” (TLR9), an immune cell receptor that recognizes invading viral and bacterial DNA.

So what does this mean for us? Well, for example, doctors have found that septic patients in hospital are most likely to die between 2 and 6 a.m. – a phenomenon that might be related to the drop in TLR9. It’s possible that patients like this could be given immune boosts during those danger hours. More generally, most of us don’t choose to get jet-lagged or miss sleep (or, to the extent we choose it, we’ve decided that the benefits outweigh the increased chance of getting sick). And we certainly don’t choose to be exposed to pathogens, either during the day or at night. Still, it’s another reminder that our bodies have evolved in a delicate balance with our environment – and when we ignore the rhythms of that environment, we leave our bodies vulnerable.